Described below is a hydraulic stroke transmitter which forwards a stroke predetermined by a solid state actuator. Generally, stroke translation is combined therewith in order to increase the originally short stroke of the solid state actuators.
For introducing a desired quantity of fuel into any combustion processes, injectors are generally required, by which a quantity of fuel may be metered. As very many combustion processes are performed by the direct injection of fuel under high pressure, frequently actuators are used which operate particularly rapidly and which drive injectors. This means that an actuator generates a stroke which, for example, actuates an injector needle which in turn opens a valve and releases fuel at predetermined time intervals and at adjustable flow rates for a combustion process. In this case, combustion air is supplied separately.
Injectors for high-pressure direct injection frequently use rapid actuators in this situation, such as for example piezoelectric multilayer actuators (PMA). The actuators are solid state actuators, the central element thereof having a plurality of piezoelectric layers. Moreover, so-called magnetostrictive solid state actuators are known which utilize a magnetic mechanical effect for generating a stroke. For generating a stroke it is important that such solid state actuators have a sufficiently small stroke in order to open an injector needle to such an extent that the desired quantity of fuel is introduced. This leads to a substantial problem, particularly in gas injectors which require a larger stroke than injectors which meter liquid fuel. As a result, only designs with a stroke translator are considered.
Where hydrogen is used as fuel, it is a drawback that the small and lightweight hydrogen molecule easily diffuses through non-metallic elements such as rubber membranes. Thus, the choice of a suitable stroke translator becomes a crucial issue in the design of the injector. This is also due to the fact that a stroke translator determines many properties of an injector and, in contrast to an actuator, may be structurally redesigned.
In previous solutions to the problem, the stroke is increased by mechanical translation or by partial hydraulic stroke translation sealed in a non-metallic manner. Mechanical stroke translators which, for example, use a mechanical lever are generally susceptible to wear and to undesirable vibrations. This applies, in particular, when an idle stroke is necessary between the actuator and the stroke translator, for example in order to prevent a leakage which could occur in the event of thermal alteration to the length due to heating. As a result, the actuator strikes against a jet needle, for example, whereby the injector is negatively affected. Uneven injection and unreliable opening and closing characteristics result. An idle stroke between the actuator and the stroke translator is also undesirable as the displacement of the actuator as far contact with the jet needle remains unexploited.
An increase of the stroke of an actuator with a transmission ratio of less than 1:2 is often implemented by mechanical levers. In injectors for diesel engines, for example, the mechanical transmission ratio may be 1:1.6. Gas injectors typically require higher transmission ratios. In gas injectors, hydraulic stroke translators, also denoted as hydraulic levers, are generally used. In the direct injection of CNG (compressed natural gas), for example, a stroke transmission ratio of 1:6 is used.
By the use of a hydraulic stroke translator, the idle stroke may be avoided so that the functional chain between the actuator and jet needle is continuously present. This is directly reflected in the structural design. In other words, the displacement of the actuator is exploited and implemented to a greater extent by the injector.
In motor vehicle technology, a drawback in the related art is, for example, the wide temperature range which has to be taken into account and which may range from −40° C. to +150° C. This may involve considerable alterations to the volume in the case of fluid volumes. Peak values may be considerably above 30% of the volume increase. For this reason, hydraulic stroke translators generally require a connection to a reservoir.
In the German published patent application DE 10 2005 042 786 A1, for example, a fuel injector is disclosed which is provided with a hermetically sealed hydraulic system. In this publication, so-called guided pistons are used. Such guided pistons require high mechanical precision in manufacture and are very susceptible to wear.